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I'm trying to figure out Python lambdas. Is lambda one of those "interesting" language items that in real life should be forgotten?

I'm sure there are some edge cases where it might be needed, but given the obscurity of it, the potential of it being redefined in future releases (my assumption based on the various definitions of it) and the reduced coding clarity - should it be avoided?

This reminds me of overflowing (buffer overflow) of C types - pointing to the top variable and overloading to set the other field values. It feels like sort of a techie showmanship but maintenance coder nightmare.

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150

Well, I don't think you'd like JavaScript much then ... and good Lord, stay away from Haskell!
– JALJan 16 '10 at 15:43

232

+1 Good question - bad assumptions (obscurity of lambda) =) Try not to be judgmental of programming techniques. Evaluate them, and add them to your mental toolkit. If you don't like them, don't use them, and be prepared to discuss them logically without becoming religious.
– KieveliFeb 23 '10 at 13:27

Love the examples, very easy to understand. But for the reduce part. If I have to implement this feature. I would do ','.join(str(x) for x in [1,2,3,4,5,6,7,8,9])
– etldsAug 30 '16 at 20:09

3

BTW if you run this on Python3 you need to call list on filter results to see the actual values, also you need to import reduce from functools
– GerardOct 20 '16 at 7:53

Are we sure about the definition of "functional programming" above? It came a little bit confusing to me.
– zgulserFeb 6 '17 at 22:53

3

I think the key point is that lambda functions can be anonymous (as they are in all your examples). If you're assigning a lambda function to anything then you're doing it wrong and should use def instead
– Chris_RandsDec 8 '17 at 14:23

1

@zgulser It's not a definition, it's just a statement about something that functional programming allows you to do.
– David ZDec 8 '17 at 22:22

lambda is just a fancy way of saying function. Other than its name, there is nothing obscure, intimidating or cryptic about it. When you read the following line, replace lambda by function in your mind:

>>> f = lambda x: x + 1
>>> f(3)
4

It just defines a function of x. Some other languages, like R, say it explicitly:

@AaronMcMillin Try type(lambda x: 3). lambda expressions and def statements both generate function objects; it is only the syntax of a lambda expression that limits which instances it can produce.
– chepnerJun 8 at 0:48

A lambda is part of a very important abstraction mechanism which deals with higher order functions. To get proper understanding of its value, please watch high quality lessons from Abelson and Sussman, and read the book SICP

These are relevant issues in modern software business, and becoming ever more popular.

Don't confuse lambdas and first-class functions. Python has an extremely limited lambda statement, but fully first-class functions. The only difference it makes is that you have to name the functions you want to pass around.
– Gareth LattyNov 18 '14 at 23:37

Curiously, the map, filter, and reduce functions that originally motivated the introduction of lambda and other functional features have to a large extent been superseded by list comprehensions and generator expressions. In fact, the reduce function was removed from list of builtin functions in Python 3.0. (However, it's not necessary to send in complaints about the removal of lambda, map or filter: they are staying. :-)

My own two cents: Rarely is lambda worth it as far as clarity goes. Generally there is a more clear solution that doesn't include lambda.

lambdas are extremely useful in GUI programming. For example, lets say you're creating a group of buttons and you want to use a single paramaterized callback rather than a unique callback per button. Lambda lets you accomplish that with ease:

This is exactly why I looked up what lambda was, but why does this work, to me it looks the exact same as just calling the function straight ( stackoverflow.com/questions/3704568/… ). Maybe it's late, it does work, but why does it work?
– RqomeyAug 5 '13 at 22:10

4

@Rqomey: the difference is that in this example value is defined in a loop; in the other example the parameter always only had one value. When you add something like arg=value, you are attaching the current value to the callback. Without it, you bind a reference to the variable in the callback. The reference will always contain the final value of the variable, since the callback happens some time after the loop has already finished running.
– Bryan OakleyAug 6 '13 at 1:56

1

I just got this working yesterday, I honestly can't believe how useful it is... I can build up a menu from one for loop and a csv file for configuration. Really useful stuff.
– RqomeyAug 7 '13 at 8:31

partial(my_callback, value) vs lambda arg=value: my_callback(arg) - the lambda has much more cruft (assignment to arg and then usage) and it's less clear what the intention is (you could be doing something subtly different in the lambda). Imports are not really a problem (you have a pile anyway and it's once per file). Code is best judged on how well it reads, and partial() is much easier to read than the lambda.
– Gareth LattyNov 19 '14 at 10:12

There is nothing you can do with lambda which you cannot do with a regular function—in Python functions are an object just like anything else, and lambdas simply define a function:

>>> a = lambda x: x + 1
>>> type(a)
<type 'function'>

I honestly think the lambda keyword is redundant in Python—I have never had the need to use them (or seen one used where a regular function, a list-comprehension or one of the many builtin functions could have been better used instead)

Yes, it's not the same, but I have never seen a cause where generating a group of lambda functions in a list has been required. It might make sense in other languages, but Python is not Haskell (or Lisp, or ...)

Please note that we can use lambda and still achieve the desired
results in this way :

>>> fs = [(lambda n,i=i: i + n) for i in range(10)]
>>> fs[3](4)
7

Edit:

There are a few cases where lambda is useful, for example it's often convenient when connecting up signals in PyQt applications, like this:

Just doing w.textChanged.connect(dothing) would call the dothing method with an extra event argument and cause an error. Using the lambda means we can tidily drop the argument without having to define a wrapping function.

your "lambda is broken" argument is broken, because of the python variable scoping rules work that way, period. You will be bitten the same way if you created a closure inside for-loop.
– Antti HaapalaFeb 27 '13 at 7:24

1

lambda is just python's way to provide the user with an "anonymous" function, like many other languages have (e.g., javascript).
– Stefan GruenwaldOct 31 '14 at 19:27

The lambda and function a's that you defined are not exactly same. :) They differ by __name__ field at least...
– progoNov 6 '14 at 7:55

It's works more than just an inline function.
– ktaNov 28 '15 at 12:23

Your argument about "might make sense in another language" is weird. Either there are intrinsic qualities of lambdas, or they aren't.
– TitouFeb 2 '17 at 15:12

If you'd have to define a function for all of those you'd go mad by the end of it. Also it wouldn't be nice with function names like plural_rule_1, plural_rule_2, etc. And you'd need to eval() it when you're depending on a variable function id.

"for the most part you should just one of those in basically any situation" Period. Typing lambda's in the interpreter isn't even that helpful.
– S.LottMay 20 '09 at 20:58

11

@Javier I agree with you if you are talking about "lambda" the concept; however, if we're talking about "lambda" the python keyword then: 1) named functions are faster and can do more (statements+expressions) almost anywhere you would use lambdas (map+filter) you can just generator expressions or list comprehensions which are more performant and concise. I'm not saying that first class functions aren't cool, just that the "lambda" keyword in python isn't as good as just using a named function, that's all.
– Aaron MaenpaaMay 20 '09 at 22:05

3

lambda has been indispensable to me for use with functions that take callback arguments like the key= argument to sort() and sorted()
– Rick CopelandMay 21 '09 at 15:57

16

@Rick I don't doubt that, but the reality is if when you see "lambda" and you think "zohmygod lambda" and start writing scheme code in python you will surly be disappointing by the limitations of python's lambda expression. On the other hand, if you start out thinking to yourself "Will a list comprehension work? No. Will what I need benifit from being a named function? No. Okay fine: sorted(xs, key = lambda x: x.name, x.height)", you will probably end up using lambda the right number of times.
– Aaron MaenpaaMay 21 '09 at 17:02

3

+1: I cannot stress that enough when one uses a lambda one uses an nameless function. And names do have a precious intellectual added value.
– Stephane RollandNov 15 '12 at 12:38

They are very useful when developing a GUI using python. Often, widgets need a reference to a function. If you need a widget to call a function and pass arguments, lambda is a very convenient way to do that.
– Bryan OakleyApr 24 '11 at 16:38

What's the advantage over passing multiple arguments into the function? func_name(a,b): return a+b rather than using lambda
– dterJul 3 '16 at 16:22

it's not needed but it helps to write shorter code and more readable in many cases, esp. in verbose languages like Java or C++
– phuclvJul 10 '17 at 3:13

I can't speak to python's particular implementation of lambda, but in general lambda functions are really handy. They're a core technique (maybe even THE technique) of functional programming, and they're also very useuful in object-oriented programs. For certain types of problems, they're the best solution, so certainly shouldn't be forgotten!

I suggest you read up on closures and the map function (that links to python docs, but it exists in nearly every language that supports functional constructs) to see why it's useful.

Now let's use map, which is the same thing, but more language-neutral. Maps takes 2 arguments:

(i) one function

(ii) an iterable

And gives you a list where each element it's the function applied to each element of the iterable.

So, using map we would have:

>>> a = [1,2,3,4]
>>> squared_list = map(lambda x: x**2, a)

If you master lambdas and mapping, you will have a great power to manipulate data and in a concise way. Lambda functions are neither obscure nor take away code clarity. Don't confuse something hard with something new. Once you start using them, you will find it very clear.

One of the nice things about lambda that's in my opinion understated is that it's way of deferring an evaluation for simple forms till the value is needed. Let me explain.

Many library routines are implemented so that they allow certain parameters to be callables (of whom lambda is one). The idea is that the actual value will be computed only at the time when it's going to be used (rather that when it's called). An (contrived) example might help to illustrate the point. Suppose you have a routine which which was going to do log a given timestamp. You want the routine to use the current time minus 30 minutes. You'd call it like so

As stated above, the lambda operator in Python defines an anonymous function, and in Python functions are closures. It is important not to confuse the concept of closures with the operator lambda, which is merely syntactic methadone for them.

When I started in Python a few years ago, I used lambdas a lot, thinking they were cool, along with list comprehensions. However, I wrote and have to maintain a big website written in Python, with on the order of several thousand function points. I've learnt from experience that lambdas might be OK to prototype things with, but offer nothing over inline functions (named closures) except for saving a few key-stokes, or sometimes not.

Basically this boils down to several points:

it is easier to read software that is explicitly written using meaningful names. Anonymous closures by definition cannot have a meaningful name, as they have no name. This brevity seems, for some reason, to also infect lambda parameters, hence we often see examples like lambda x: x+1

it is easier to reuse named closures, as they can be referred to by name more than once, when there is a name to refer to them by.

it is easier to debug code that is using named closures instead of lambdas, because the name will appear in tracebacks, and around the error.

That's enough reason to round them up and convert them to named closures. However, I hold two other grudges against anonymous closures.

The first grudge is simply that they are just another unnecessary keyword cluttering up the language.

The second grudge is deeper and on the paradigm level, i.e. I do not like that they promote a functional-programming style, because that style is less flexible than the message passing, object oriented or procedural styles, because the lambda calculus is not Turing-complete (luckily in Python, we can still break out of that restriction even inside a lambda). The reasons I feel lambdas promote this style are:

There is an implicit return, i.e. they seem like they 'should' be functions.

They are an alternative state-hiding mechanism to another, more explicit, more readable, more reusable and more general mechanism: methods.

I try hard to write lambda-free Python, and remove lambdas on sight. I think Python would be a slightly better language without lambdas, but that's just my opinion.

"because the lambda calculus is not Turing-complete" is plain wrong, untyped lambda calculus IS Turing complete, that is the reason why it is so significant. You can get recursion using the Y-combinator, Y = lambda f: (lambda x: x(x))(lambda y: f(lambda *args: y(y)(*args)))
– Antti HaapalaFeb 27 '13 at 7:50

Furthermore, if one goes to Wikipedia to read on Turing completeness, it says "A classic example is the lambda calculus."
– Antti HaapalaFeb 27 '13 at 7:56

6

Good Lord, answer that claims lambda-calculus is not Turing-complete with 10 upvotes?!
– Marcin ŁośSep 10 '13 at 13:52

Lambdas are actually very powerful constructs that stem from ideas in functional programming, and it is something that by no means will be easily revised, redefined or removed in the near future of Python. They help you write code that is more powerful as it allows you to pass functions as parameters, thus the idea of functions as first-class citizens.

Lambdas do tend to get confusing, but once a solid understanding is obtained, you can write clean elegant code like this:

squared = map(lambda x: x*x, [1, 2, 3, 4, 5])

The above line of code returns a list of the squares of the numbers in the list. Ofcourse, you could also do it like:

def square(x):
return x*x
squared = map(square, [1, 2, 3, 4, 5])

It is obvious the former code is shorter, and this is especially true if you intend to use the map function (or any similar function that takes a function as a parameter) in only one place. This also makes the code more intuitive and elegant.

Also, as @David Zaslavsky mentioned in his answer, list comprehensions are not always the way to go especially if your list has to get values from some obscure mathematical way.

From a more practical standpoint, one of the biggest advantages of lambdas for me recently has been in GUI and event-driven programming. If you take a look at callbacks in Tkinter, all they take as arguments are the event that triggered them. E.g.

Now you can argue that this can be done using global variables, but do you really want to bang your head worrying about memory management and leakage especially if the global variable will just be used in one particular place? That would be just poor programming style.

In short, lambdas are awesome and should never be underestimated. Python lambdas are not the same as LISP lambdas though (which are more powerful), but you can really do a lot of magical stuff with them.

Thanks. I completely didn't understand your last example. How come x and y are defined in both main and do_something_cool? What happens to x and y in the function? The values passed seem to be immediately overwritten? How does the function know about event? Could you add some comments / explanation? Thanks
– Sanjay ManoharApr 9 '15 at 21:03

@SanjayManohar I am passing x and y as arguments to do-something-cool and their values are being set in the function to illustrate how you can use lambdas to pass arguments where none are expected. The widget.bind function expects a event parameter which identifies the GUI event on that particular widget. I recommend reading on Tkinter's programming model for greater clarity.
– varagrawalApr 10 '15 at 5:08

hmm I think I understand the Tkinter model. But still don't quite understand -- you pass x,y then do x=event.x. Doesn't that overwrite the value you passed? And how does the function know what event is? I don't see where you pass that to the function. Or is it a method? Also are you allowed minus signs in a function name?
– Sanjay ManoharApr 10 '15 at 18:37

@SanjayManohar buddy you need to read up on Tkinter and Python. Tkinter passes the event object the function as a default action. As for the x, y, that is just an example for illustrative purposes. I am trying to show the power of lambdas, not Tkinter. :)
– varagrawalApr 15 '15 at 14:48

1

OK now I see what you intended - you want the function to receive event? in that case, shouldn't your lambda read lambda event: do_something_cool(event,x,y) ?
– Sanjay ManoharApr 16 '15 at 21:57

Lambdas are deeply linked to functional programming style in general. The idea that you can solve problems by applying a function to some data, and merging the results, is what google uses to implement most of its algorithms.

Programs written in functional programming style, are easily parallelized and hence are becoming more and more important with modern multi-core machines.
So in short, NO you should not forget them.

First congrats that managed to figure out lambda. In my opinion this is really powerful construct to act with. The trend these days towards functional programming languages is surely an indicator that it neither should be avoided nor it will be redefined in the near future.

You just have to think a little bit different. I'm sure soon you will love it. But be careful if you deal only with python. Because the lambda is not a real closure, it is "broken" somehow: pythons lambda is broken

Python's lambda is not broken. There are two ways to handle locals in lambdas. Both have advantages and disadvantages. I think the approach Python (and C#) took is probably counterintuitive to those more accustomed to purely functional languages, since with a purely functional language I don't think that approach even makes sense.
– BrianMay 20 '09 at 20:54

It is indeed counterintuitive. I'm not a python programmer but in squeak smalltalk it is the same and I stumple upon this regularly. So even I would consider it "broken" :)
– Norbert HartlMay 20 '09 at 21:02

No, this has nothing to do with counterintuitivity. It is just that the lexical scope of variable names is that of a function; a loop does not introduce a lexical scope. Functions also work the same way in Javascript. If you need a scope for var, you can always do (lambda scopedvar: lambda x: scopedvar + x)()
– Antti HaapalaFeb 27 '13 at 7:26

I'm just beginning Python and ran head first into Lambda- which took me a while to figure out.

Note that this isn't a condemnation of anything. Everybody has a different set of things that don't come easily.

Is lambda one of those 'interesting' language items that in real life should be forgotten?

No.

I'm sure there are some edge cases where it might be needed, but given the obscurity of it,

It's not obscure. The past 2 teams I've worked on, everybody used this feature all the time.

the potential of it being redefined in future releases (my assumption based on the various definitions of it)

I've seen no serious proposals to redefine it in Python, beyond fixing the closure semantics a few years ago.

and the reduced coding clarity - should it be avoided?

It's not less clear, if you're using it right. On the contrary, having more language constructs available increases clarity.

This reminds me of overflowing (buffer overflow) of C types - pointing to the top variable and overloading to set the other field values...sort of a techie showmanship but maintenance coder nightmare..

Lambda is like buffer overflow? Wow. I can't imagine how you're using lambda if you think it's a "maintenance nightmare".

I started reading David Mertz's book today 'Text Processing in Python.' While he has a fairly terse description of Lambda's the examples in the first chapter combined with the explanation in Appendix A made them jump off the page for me (finally) and all of a sudden I understood their value. That is not to say his explanation will work for you and I am still at the discovery stage so I will not attempt to add to these responses other than the following:
I am new to Python
I am new to OOP
Lambdas were a struggle for me
Now that I read Mertz, I think I get them and I see them as very useful as I think they allow a cleaner approach to programming.

He reproduces the Zen of Python, one line of which is Simple is better than complex. As a non-OOP programmer reading code with lambdas (and until last week list comprehensions) I have thought-This is simple?. I finally realized today that actually these features make the code much more readable, and understandable than the alternative-which is invariably a loop of some sort. I also realized that like financial statements-Python was not designed for the novice user, rather it is designed for the user that wants to get educated. I can't believe how powerful this language is. When it dawned on me (finally) the purpose and value of lambdas I wanted to rip up about 30 programs and start over putting in lambdas where appropriate.

A useful case for using lambdas is to improve the readability of long list comprehensions.
In this example loop_dic is short for clarity but imagine loop_dic being very long. If you would just use a plain value that includes i instead of the lambda version of that value you would get a NameError.

I can give you an example where I actually needed lambda serious. I'm making a graphical program, where the use right clicks on a file and assigns it one of three options. It turns out that in Tkinter (the GUI interfacing program I'm writing this in), when someone presses a button, it can't be assigned to a command that takes in arguments. So if I chose one of the options and wanted the result of my choice to be:

print 'hi there'

Then no big deal. But what if I need my choice to have a particular detail. For example, if I choose choice A, it calls a function that takes in some argument that is dependent on the choice A, B or C, TKinter could not support this. Lamda was the only option to get around this actually...